Field of the Invention
The invention relates to helium re-liquefaction. The invention relates to an apparatus and a method for helium collection and re-liquefaction in a magnetoencephalography measurement device.
Description of the Related Art
In Magnetoencephalography the magnetic fields generated by the brain activity of a patient are measured using Superconducting Quantum Interference Devices (SQUIDs), which are very sensitive magnetometers. The SQUID magnetometers require a working temperature close to 4 Kelvins. To achieve this working temperature, in a Magnetoencephalography (MEG) measuring device the SQUID magnetometers are placed inside a specially designed vacuum insulated Dewar vessel containing Liquid Helium (LHe). The SQUID magnetometers in the Dewar vessel are used to form a sensor assembly, which conforms to the shape of the patient's head. For the head there is an appropriately shaped helmet in the body of the Dewar vessel, in order to allow positioning of the sensor assembly as close to the patient's brains as possible. The helium in the Dewar vessel is liquefied elsewhere and transferred to the MEG measuring device using separate storage Dewar vessels and/or vacuum insulated transfer lines. In Magnetoencephalography the magnetic fields to be measured are extremely weak. In brains the magnetic fields are caused by a plurality of synchronized neuronal currents together inducing a net magnetic field that can be detected outside the patient's skull. The net magnetic field strength varies between 10 fT observed for cortical activity and 1000 fT observed for the alpha rhythm during wakeful relaxation with closed eyes of the patient. The weakness of the magnetic fields to be measured using a MEG measurement device is a problem when considering that there are usually several types of disturbing non-interesting magnetic fields which are due to interference sources at various distances from the MEG measurement device. The non-interesting magnetic fields may be due to, for example, supply lines for electrical power and a variety of electrical apparatuses. In order to alleviate the problem, the MEG measurement device is usually installed into a magnetically shielded room. The magnetically shielded room achieves a dampening of ambient magnetic fields outside the room, for example, by a factor of 102 to 106 depending on the oscillation frequency of the magnetic fields.
However, the magnetically shielded room does not help for interference sources located inside the magnetically shielded room. Examples of such interference sources include electrical apparatuses associated with the MEG measurement device itself. Liquid Helium (LHe) in the Dewar vessel boils off gradually due to the operating of the MEG measurement device and the thermal leakage into the Dewar vessel from the room temperature. The boiled helium may be lead outside the Dewar vessel and let into the atmosphere. This is not a preferable solution since helium is a scarce and costly resource. There is a problem if Helium needs to be re-liquefied in the Dewar vessel. The re-liquefying must be performed in the Dewar vessel in order to avoid using long Liquid Helium (LHe) supply lines, which are a complicated and an expensive solution. A solution is to have a cryocooler that liquefies helium directly into the Dewar vessel. The problem is that cryocoolers such as Gifford-McMahon cryocoolers, pulse tube cryocoolers, and Stirling cryocoolers cause electromagnetic interference which disturbs the measurement of weak interesting magnetic fields. It would be in principle possible to compensate in association with measurement signal processing for the electromagnetic noise caused by the cryocooler by utilizing knowledge of the electrical characteristics of the cryocoolers such as locations of the electrical wiring. However, such solutions are complicated, require large dynamic range of the SQUID sensors, and diminish the reliability of the magnetic field measurements of the MEG device. A further problem is the vibration caused by the mechanical movement of the displacer in the cold head of the GM cryocooler, or the operation of the rotary valve in the pulse-tube coolers. Similar mechanical movements are present in Stirling cryocoolers. Therefore, it would be beneficial to have a solution for re-liquefaction of helium which does not complicate MEG measurement result analysis and does not reduce the reliability of magnetic field measurements in a MEG measurement device.